The present invention relates to compositing of video images, and more particularly to a chroma keyer with fringe control offset which generates a hue select signal that responds to areas of a foreground object where there is mixing of a background hue with the foreground object within a foreground image.
In television it is often desirable to superimpose a foreground object, such as a weather man, in front of a background image, such as a weather map. The foreground object is placed in front of a reference background of a specified hue, usually blue, to form a foreground image. The foreground image is input to a chroma keyer to generate a key signal that cuts a hole in the background image to form a shaped background image while suppressing the reference background from the foreground image to isolate the foreground object as a shaped foreground image. The shaped foreground image is then composited with the shaped background image to produce the illusion of the foreground object appearing in front of the background image.
Traditional implementations of chroma keying circuits have not always been able to produce results that are free from unwanted artifacts. One such artifact is caused by the mixing of the hue in the reference background with the hues of the foreground object, particularly around the fringes of the foreground object. For example a blue reference background may be reflected from or transmitted through part of the foreground object such that the color of the foreground object, especially at the edges, is changed or removed. As seen by a video camera brown hair may become mixed with the blue background so that the hair is not brown enough, or due to vector color addition is grey, i.e., has luminance values only. This mixing seems normal when the foreground object is in front of the reference background, but when a new background image replaces the reference background a grey halo may appear around the foreground object, giving an unnatural appearance.
There are ways to correct the hue of the foreground object to remove the hue of the reference background from the foreground object. One method is to remove some "blue", i.e., the hue of the reference background, from everywhere in the foreground object. However uniform removal of "blue" from the foreground object does not work well since the mixing of the hue from the reference background with the hues of the foreground object is not uniform, but occurs especially at the edges and in transparent parts of the foreground object. Another method is to remove "blue" from one specific mix of colors. However this method is limited by being able to correct only one mix of colors, and cannot correct a mix that produces no color at all.
A typical chroma keyer circuit is implemented in the Model 3000 Production Switcher, manufactured by The Grass Valley Group, Inc. of Grass Valley, California, United States of America and described in U.S. Pat. No. 5,251,016 issued Oct. 5, 1993 to James A. Delwiche entitled "Chroma Keyer with Secondary Hue Selector." A hue selector, shown in detail in FIG. 4 of the Delwiche application, compares the hues of the foreground image, having the foreground object in front of the reference background, with the hue of the reference background to generate a hue select signal. The hue select signal, as shown in FIG. 1 of the Delwiche application, is input to a key circuit to generate the chroma key signal. The hue select signal also is used to shape a color matte signal from a color matte generator via a multiplier in such a way that, when subtracted from the foreground image, it produces a shaped foreground image having black in place of the reference background. The shaped foreground image is then composited with the background image, as shaped by the chroma key signal, to produce the composited image. A negative clipper at the output of the hue selector assures that the hue select signal only has positive values.
Regions of the foreground image where the colors mix to form grey appear as a zero level in the hue select signal. Colors of other hues which do not correlate with the hue of the reference background, according to the selectivity of the hue selector, and produce negative values are clipped at zero. Colors having hues correlated with the hue of the reference background produce positive values as the hue select signal.
What is desired is a chroma keyer that corrects color mixing between the reference background and the foreground object, especially when the color mixing produces a region of the foreground object having luminance values only.